Don't forget your running two GPU'S so doe's the grunt factor really matter. l'v just gone Sli 670's 4gb and run them at stock Core's at 915mhz, from 6970.
Every game l'v tried is so smooth, got 4gb to be more future proof all round, just got a Samsung 971D 27" 2560x1440res monitor as a Xmas prezzy of the family.
Been playing Ghost at 2560x1440res prior to that 24" 1920x1080res, and it really brings out the colours and the map detail.
My advice is go for what you can afford minimum Nvidia 670 miss out the 760, 770 Sli and AMD 7950, 7970 Xfire.
Single 780 or 780Ti, add another if you want in the future, as the R29 seems to have Black Screen issue's.
Black Screen thread >
http://forums.overclockers.co.uk/showthread.php?t=18557629